June 7,1924 
Variation in Microtus Montanus Yosemite 
999 
other families and subfamilies, however, height of the cranium can not satis¬ 
factorily be taken in just this manner, but special methods must be devised, as 
height from the pterygoid plates, or measured from the alveolar border between 
the penultimate and last molars. A scrutiny of the series at hand shows that 
the actual height of the posterior portion of the braincase differs hardly at all 
Fig. 14—Graph for skull measurements of the female series, reduced to percentages, with the largest skull 
considered as par, and arranged according to condylo-basflar sequence 
with age, although there is some individual variation in this measurement. 
The difference with age shown in .the tables is caused chiefly by variation in 
the infero-superior dimension of the bullae, but the height measured from the 
inferior border of the foramen magnum is almost uniform. Unfortunately, 
without calipers of special design it is practically impossible to measure the 
height of skull after this fashion with a satisfactory degree of uniformity. 
Zygomatic width. —The greatest spread of the zygomatic arches. The 
average of this measurement for males is 16 mm., with extremes of 17.9 and 14.4, 
giving a variation of 3.5 mm., or 21.9 per cent. For females the average is 15.63 
mm., with extremes of 17.6 and 14.6, a difference of 3 mm., or 19.2 per cent. 
